Generally, a handover function is as follows. According to a handover, although a mobile terminal (also called a user equipment (UE)) making a phone call to another mobile terminal in a communication zone (e.g., a mobile communication cell) moves from an area of one base station (also called an e-nodeB) to an area of another base station (i.e., another eNode-B), a handover from one base station to another base station is carried out, such that the mobile terminal can continuously keep making the phone call to another mobile terminal.
The handover occurs when a radio frequency (RF) channel currently used by a mobile station subscriber (i.e., UE) is in poor condition, or also occurs when the mobile station subscriber UE moves from a current eNode-B area to another eNode-B area.
The handover may be classified into a softer handover, a soft handover, and a hard handover according to channel switching methods.
The softer handover is carried out in one cell, and may indicate that a channel used by the UE is switched to a good channel from among channels of a UE's cell coverage. The soft handover indicates that two neighboring channels are simultaneously operated and finally one of the two neighboring channels is slowly severed. According to Code Division Multiple Access (CDMA) scheme, two or more eNode-Bs use the same frequency band, such that this soft handover can be easily implemented in the CDMA scheme. According to the hard handover, a current call-connected channel is severed, and is immediately connected to another channel.
According to the entity carrying out the above-mentioned handover, the handover may be classified into a Network Controlled Handoff (NCHO), a Mobile Assisted Handoff (MAHO), a Mobile Controlled Handoff (MCHO), etc.
The soft handover and the MAHO from among the above-mentioned schemes will hereinafter be described in detail. Needless to say, another handover method may also be used.
An Intra Radio Access Technology (I-RAT) handover supports a handover between homogeneous networks. If a UE moves from one eNode-B (i.e., a serving eNode-B) to another eNode-B (i.e., a target eNode-B) within the range of a homogeneous network, the I-RAT handover provides this UE with a seamless handover and service continuity, resulting in greater convenience of a user of the UE.
FIG. 1 is a flow chart illustrating a handover and initial network entry process.
Referring to FIG. 1, a UE enters a handover and initial network entry process, and selects a cell at steps S101 and S102. During this cell selection, the UE performs a scanning or ranging process together with one or more eNode-Bs in order to search for a desired eNode-B appropriate for either a network connection or a handover. The UE must perform scheduling either the scanning or sleep period such that it can determine whether or not the UE enters an initial network of the eNode-B or is handed over to a target eNode-B (T-eNB).
When the UE enters the initial network, it establishes synchronization with a serving station (also called a serving eNode-B) and a downlink parameter at step S103. The serving eNode-B (S-eNB) provides services over a network to which the UE will belong. The UE synchronized with the serving eNode-B (S-eNB) acquires an uplink parameter for the serving eNode-B (S-eNB) at step S104. Then, the UE performs the ranging process along with the serving eNode-B (S-eNB) and adjusts uplink parameters for the serving eNode-B (S-eNB) at step S105. By the above-mentioned steps, the UE and the serving eNode-B (S-eNB) form a basic function for communication at step S106. The serving eNode-B (S-eNB) provides the UE and exchanges keys with the UE at step S107. Thus, the UE is registered in the serving eNode-B (S-eNB) at step S108, and establishes an IP connection at step S109.
The serving eNode-B (S-eNB) transmits operation parameters to the UE such that the UE performs a communication process at step S110. If a connection between the UE and the serving eNode-B (S-eNB) is established at step S111, the UE and the serving eNode-B can be normally operated at step S112. While the serving eNode-B (S-eNB) is normally operated, the UE continuously searches for a neighboring eNode-B at step S113, because a quality of service (QoS) is lowered in proportion to the distance from the UE to the serving eNode-B (S-eNB). Therefore, the neighboring eNode-B providing services better than those of the serving eNode-B (S-eNB) is called a target station (i.e., a target eNode-B), such that it can perform the handover to the target eNode-B.
Generally, the handover is carried out when the UE moves from the serving eNode-B (S-eNB) to the target eNode-B (T-eNB). The handover indicates that a wireless interface, service flow, and network access point of the UE are handed over to the target eNode-B (T-eNB). When the UE, the serving eNode-B (S-eNB), or the network administrator determines the handover at step S114, this handover begins.
The UE selects the target eNode-B (T-eNB) at step S115, establishes synchronization with the target eNode-B (T-eNB) and acquires downlink parameters at step S116. The UE acquires uplink parameters of the target eNode-B (T-eNB) at step S117, performs the ranging process together with the target eNode-B (S-eNB) and adjusts uplink parameters at step S118. In this case, if the UE pre-receives an identifier, frequency, and uplink/downlink channel descriptors (UCD/DCD) of the target eNode-B (T-eNB), the scanning process and the synchronization process can be simplified. If the target eNode-B (T-eNB) receives a handover notification message from the serving eNode-B (S-eNB) over a backbone network, a non-competitive initial ranging opportunity can be provided to an UL-MAP.
By the above-mentioned steps, the target eNode-B (T-eNB) forms the basic function at step S119, and the UE and the target eNode-B (T-eNB) perform the ranging and begins to perform the network re-entry process. The UE is re-registered in the target eNode-B (T-eNB), and is re-connected to the target eNode-B (T-eNB) at step S120. Thus, the terminal is registered in the target eNode-B (T-eNB) at step S121. The IP connection of the target eNode-B (T-eNB) is re-established in the UE at step S122. By the above-mentioned steps, the target eNode-B (T-eNB) serves as the serving eNode-B (S-eNB), and can provide the UE with necessary services.